This invention relates generally to life testing and, more particularly, to apparatus and methods for measuring frequency and displacement for a high cycle fatigue closed loop control.
Life testing is typically utilized in connection with components used in applications that are subjected to wide temperature ranges and vibration amplitudes. A high cycle fatigue (HCF) portion of the life test uses a closed loop control to measure frequency and displacement of the components. The component being tested is installed on a test stand and. shaken with an oscillator in an open loop mode. A capacitance probe is positioned next to the stand and measures a frequency and vibration amplitude of the component as it is shaken. Specifically, the capacitance probe measures the separation between the probe and a target material by determining an existing capacitance through a relationship between a charge and a potential difference between the two surfaces.
During the testing, as the component fatigues, a natural resonant frequency and amplitude of the component changes and the capacitance probe generates a signal representative of such changes. The signal generated by the capacitance probe is provided to the shaker closed loop control, and the shaker closed loop control operates to maintain the component vibration amplitude to a pre-set amplitude as the component fatigues.
Known capacitance probes include an internal oscillator and a receiver circuit. Because different components are tested, often the internal oscillator is a free-running multi-vibrator circuit capable of transmitting various frequencies. However, because the oscillator is a free-running circuit, the frequency transmitted by the oscillator may drift during testing depending on a proximity of the capacitance probe to the component being tested. In addition, due to the sensitivity of the receiver circuit, undesirable noise and other signals transmitted by the oscillator or generated during testing are received by the receiver circuit and may adversely impact the testing.
Furthermore, because the frequency drifts depending on the proximity of the capacitance probe to the component, DC voltage shifts occur depending on the proximity of the capacitance probe to the component. Such voltage shifts limit a dynamic range of output voltage of the capacitance probe and a wide band width and sensitivity of the receiver may permit electrical noise to dominate the frequencies being received.
In an exemplary embodiment, a testing method permits frequency and displacement for a high cycle fatigue life test of a component to be accurately measured. More specifically, and in one embodiment, a capacitance probe accurately detects, measures and tracks the-frequency and amplitude changes of the component during the testing in a closed loop control. The probe includes an internal oscillator, a receiver circuit, a product detector, and a pair of gain amplifiers. The oscillator generates a single controlled frequency output received by the adjustable receiver circuit. The receiver circuit includes a capacitance bridge and is also adjustable to create a DC voltage shift that effectively nulls the capacitance bridge. The product detector detects an amplitude modulated product of the oscillator and the receiver circuit, and rejects undesirable frequencies before relaying a signal from the product detector. The gain amplifiers detect a DC voltage shift and permit the capacitance effect to be easily nulled. The gain amplifiers are also adjustable to prevent over-driving or over-amplifying of the output of the capacitance probe.
During a high cycle fatigue portion of a life test of the component, the capacitance probe is placed adjacent the component and is used to detect and track the frequency and amplitude changes of the component. During the test, as the component fatigues and the natural resonant frequency decreases, the capacitance probe transmits the changes in amplitude to the shaker closed loop control. Electric noise generated during the testing is eliminated with the use of the product detector and the gain amplifiers are adjusted to null the capacitance effect and permit a dynamic range of the output signal to be increased and centered such that the capacitance probe closed loop is stable. As a result, the capacitance probe provides accurate measurements of the frequency and displacement of a component during high cycle fatigue testing in a more accurate and cost-effective manner in comparison to known capacitance probes.